This invention concerns a method for repairing or protecting a damaged or corroded area of the end of a metal heat exchanger tube, near the inside surface of its tube sheet, whereby a metal sleeve having an outside diameter slightly smaller than the tube's inside diameter is inserted into the tube to a sufficient depth to cover the damaged or corroded area, the end of the sleeve is engaged against the tube wall beyond the damaged or corroded area thereof, then the engaged end of the sleeve is locally welded to the inside of the tube such that the latter is partially penetrated and the end of the sleeve towards the outside surface of the tube sheet and the corresponding end of the tube is locally welded. The invention also concerns a sleeve which can be used with this method.
In heat exchangers made of tube bundles attached to thick tube sheets, one of the fluids, for example the heating fluid, flows inside the tubes, and the other, for example the heated fluid, circulates outside the tubes.
When, as a result of corrosion, a defect wears through the wall or walls of one or more tubes, causing even a small leak of one fluid into the other, several disadvantages appear, namely:
loss of exchanger effectiveness, and consequently of the installation's efficiency, PA1 the risk of defect growth, to the point of complete rupture of the tube, which may then whip about and damage neighboring tubes, PA1 and in the case of a nuclear plant exchanger, especially in pressurized water reactor (PWR) steam generator boilers, possible leakage of radioactive fluid into nonradioactive fluid and contamination of the latter's circuit. PA1 Prior to inserting the sleeve into the tube, drill one or more small-diameter blind holes or through holes into the tube, slightly beyond the inside surface of the tube sheet. PA1 Locally bulge or expand the end of the sleeve towards the outside surface of the tube sheet in the open air, or in an inert gas atmosphere or with blown inert gas. PA1 Prior to inserting the sleeve into the tube, provide the end of the former intended to be expanded the tube near the outside of the tube sheet with a thin metallic coating which is a good heat conductor and is corrosion resistant. PA1 Prior to inserting the sleeve into the tube, provide the end of the former intended to be expanded against the tube near the outside of the tube sheet with a series of longitudinal grooves, and before making the weld, purge the air from the space between the tube and the sleeve by blowing inert gas through the grooves or vacuum sucking the grooves. PA1 Also provide longitudinal grooves in the other end of the sleeve and flush the air from the space between the tube and sleeve with inert gas or vacuum sucking. PA1 After welding the sleeve to the tube end towards the outside of the tube sheet, carry out several expansion passes to bulge the sleeve into the tube beyond the weldment. PA1 Weld the sleeve to the tube inwards of the ends of the sleeve. PA1 Engage the end of the sleeve with the tube beyond the damaged or corroded area by controlled deformation of the sleeve's diameter. PA1 Weld the sleeve to the tube beyond the damaged or corroded area of the tube by welding the edge of the sleeve itself. PA1 De-stress the weldment securing the bulged end of the sleeve to the inside of the tube beyond the damaged or corroded are using an internal inductor.
It has previously been proposed to eliminate to leak in a tube wall by plugging the two ends of the tube in the tube sheet or sheets. This can be done only for a limited number of tubes however, to avoid substantially reducing the flow of fluid processed in the exchanger.
Another proposed solution has been to stop the leak with the help of an internal sleeve installed either by mechanical or hydrostatic bulging, or by expansion with an elastomer ring, by explosive forming or by brazing.
Such methods are not fully satisfactory however, for securing the sleeve within the tube by mechanical or hydrostatic bulging, expansion with an elastomer ring, explosive forming or brazing induces stresses in the tube which are potential sources of corrosion that cannot be eliminated without the risk of spring-back or "debulging".